Photodetection apparatuses are known that employ the CMOS (Complementary Metal Oxide Semiconductor) technique. Among these, in particular, those employing an active pixel method are well known (see, for example, Patent Document 1). Such a photodetection apparatus of the active pixel method comprises an active pixel type pixel section including a photodiode for generating electric charge of an amount corresponding to incident light intensity, and thereby performs charge-voltage conversion on the electric charge generated by the photodiode in correspondence to the light incidence in the pixel section, via a source follower circuit composed of a transistor. This photodetection is achieved with high sensitivity and low noise.
When the charge accumulation capacitance value is denoted by Cf in a floating diffusion region for accumulating the electric charge generated by the photodiode in the pixel section, and when the amount of the electric charge is denoted by Q, the output voltage value V acquired by the charge-voltage conversion is expressed by a formula V=Q/Cf. As seen from this formula, when the charge accumulation capacitance value Cf of the floating diffusion region is reduced, the sensitivity can be increased in the photodetection.
Patent Document 1: Japanese Patent Laid-Open publication No. Hei-11-274454
Nevertheless, the output voltage value V is restricted to a few V at maximum owing to the available range of the supply voltage and various limitations in the circuit system. Further, the amount Q of electric charge that can be accumulated in the floating diffusion region also has an upper limit. This also places a restriction on the output voltage value V.
In order to increase the upper limit (saturation electric charge amount) of the amount Q of electric charge which can be accumulated in the floating diffusion region, the charge accumulation capacitance value Cf of the floating diffusion region may be increased, or alternatively the supply voltage value may be increased. Nevertheless, the increasing of the capacitance value Cf of the floating diffusion region requires the reducing of the supply voltage value. As a result, no increased saturation electric charge amount is obtained. Further, when the charge accumulation capacitance value Cf of the floating diffusion region is increased, the remarkable advantage of high sensitivity is lost.
As such, a prior art photodetection apparatus can perform photodetection with high sensitivity, but has the disadvantage of a narrow dynamic range of the photodetection caused by the restriction in the saturation electric charge amount.